In recent times, a secondary battery refers to a device which converts external electric energy into the form of chemical energy and stores it therein, and then generates electricity as necessary. Such a secondary battery is also called the name of ‘rechargeable battery’ which means a battery capable of being charged many times. Typical secondary batteries include a lead storage battery, nickel cadmium (Ni—Cd) battery, nickel metal hydride (NiMH) battery, lithium ion (Li-ion) battery and a lithium ion (Li-ion) polymer battery. A secondary battery provides both an economical advantage and an environmental advantage as compared to a disposable primary battery.
Currently, a secondary battery is used for some applications requiring low electric power. For example, such applications include a device that helps starting of a car, portable system, instrument and a no-brake power supply system. Recently, development of wireless communication technology leads popularization of a portable system. In addition, there is a tendency to convert many conventional systems into wireless systems. Under these circumstances, there is an exploding demand for secondary batteries. Further, hybrid cars and electric cars have been commercialized with a view to preventing environmental pollution. Such next-generation vehicles adopt secondary batteries to reduce the cost and weight and to increase the service life.
In general, secondary batteries are generally provided as cylindrical, prismatic or pouch-type batteries. This is because secondary batteries are manufactured by installing an electrode assembly including a negative electrode, positive electrode and a separator into a cylindrical or prismatic metal can or a pouch-type case made of an aluminum laminate sheet, and then injecting an electrolyte to the electrode assembly. Therefore, a predetermined space for installing a secondary battery is required essentially. Thus, such cylindrical, prismatic or pouch-like shapes of secondary batteries undesirably function as limitations in developing various types of portable systems. As a result, there is a need for a novel type of secondary battery which allows easy deformation.
To satisfy such a need, a linear battery, which has a significantly larger ratio of length to sectional diameter, has been suggested. Korean Laid-Open Patent No. 2005-99903 discloses a variable battery including an inner electrode, an outer electrode and an electrolyte layer interposed between both electrodes. However, such a battery has poor flexibility. In addition, the linear battery uses a polymer electrolyte to form an electrolyte layer, and thus makes it difficult to inject an electrolyte into an electrode active material, thereby causing an increase in resistance of the battery and degradation of capacity and cycle characteristics.
Additionally, when forming a cable-type secondary battery, a non-uniform gap is generated between the inner electrode/the outer electrode and a separator layer interposed therebetween. However, such a gap prohibits smooth injection of an electrolyte to the active material layer of the outer electrode, thereby causing a problem of degradation of battery performance
Further, when using a wire-type current collector in a cable-type secondary battery, a linear resistance is generally higher than a sheet resistance. Thus, a wire-type current collector has higher resistance characteristics as compared to a sheet-type current collector, thereby causing degradation of battery performance. On the contrary, a sheet-type current collector has relatively lower strength, thereby causing a problem of cutting.
Moreover, when manufacturing a cable-type secondary battery by using a sheet-type current collector, a process of winding each of sheet-type electrodes and separators has to be undergone, resulting in an increase in processing time and a difficulty in connecting a current collector with an electrode tab.